* Now IpPrefix uses IpAddress to represent the subnet address
* The IpPrefix subnet address is masked-out by the prefix length.
E.g., IpPrefix("1.2.3.4/24") is now stored as IpPrefix("1.2.3.0/24")
* Removed IpPrefix methods that are not used or don't apply anymore
* Replaced usage of IpPrefix with IpAddress where appropriate
* Updated classes PortAddresses (in core/api) and Interface (in sdnip)
to use class InterfaceIpAddress instead of IpPrefix
* Updated corresponding unit tests and relevant code
* Minor refactoring inside ProxyArpManager to simplify some of the
logic and usage related to PortAddresses. Also, renamed
method findOutsidePortInSubnet() to findPortInSubnet() and updated
its implementation to reflect better its usage.
